Chapter 5. Work, energy and power

Question 1

What is power

Answer 1

The rate at which work is done

Question 2

What is the law of conversation of energy

Answer 2

Energy can neither be created nor destroyed but can only be converted from one form to another

Question 3

Note

Answer 3

Work is a scalar

Question 4

What is the formula for work

Answer 4

w(J) = force,F(N) x displacement,s(m)
work is the product of force and the displacement in the same direction
work is the amount of energy transfered from one form to another
work is a scalar

Question 5

Define a joule

Answer 5

The work done when a force of 1N moves through a distance of 1m in the direction of the force

Question 6

What is power?

Answer 6

The rate at which work is done.
Power,P = work done,W / time taken,t = energy transfered,E/time taken,t
1 watt = 1 joule per second
P = Fv

Question 7

What is the formula for kinetic energy?

Answer 7

E_k = 1/2 x mass x velocity²

Question 8

What is the formula for gravitational potential energy?

Answer 8

E_p = mass x gravitational strength x change in height

Question 9

Deriving the formulas

Answer 9

For KE
make a subject of formula in v² = u² + 2as when u = 0 then substitute it into work = Fs=mas
For PE
work = Fs = mas = mgh

Question 10

What is internal energy?

Answer 10

The total kineticand potential energy of all the atoms or molecules of a substance

Question 11

What is the principle of conservation of energy?

Answer 11

Energy cannot be created or destroyed but can only be transfered from one form to another.

Question 12

What is a machine?

Answer 12

Any device that transfers energy from one form to another.

Question 13

What is efficiency?

Answer 13

The proportion of enrgy thatis usefully transferred
efficiency = useful energy output/total energy input = useful power output/total, power input as a percentage

Question 14

What is the formula for E_k in terms of momentum?

Answer 14

E_k = p²(momentum) / 2m(mass)
from the formula of momentum make v subject of formula and substitute v in the formula for E_k

Question 15

Note

Answer 15

Momentum is always conserved
In elastic collisions the kinetic energy is the same before and after the collision
In inelastic collisions the kinetic energy before the collision is more than the kinetic energy after the collision as the kinetic energy is lost as interal energy

Question 16

Different forms of energy

Answer 16

-gravitational potential energy, which is the energy due to position in a gravitational field
-kinetic energy, the energy due to motion
-electrical potential energy, which is the energy of electrically charged objects,
-elastic potential energy, which is the energy stored in an object when it is compressed or stretched.

Question 17

Note

Answer 17

The area under a pressure volume graph is equal to work done

Question 18

What is elastic potential energy?

Answer 18

The enrgy stored in a spring obeying Hooke’s law
1/2 ke²
On a Force extension graph the area under the graph is equal to the work done